Before MetroMPG yells 'It depends!'(tm), let me say that generally it's better to block the upper intake. The opening above the bumper is often a styling point, and not a function-driven design. Blocking the upper opening helps deflect smoother air up and over the vehicle. Still speaking in broad generalities, the lower opening is often oversized and can be at least partially blocked. I would recommend to first install an accurate ECT gauge or use a Scangauge to monitor coolant temperature. Then progressively block your grill until your comfort zone is violated.
What flavor of vehicle are we talking about here? Got pictures?